Imparting a wave to hair generally involves rendering the hair pliable, imparting the curl, such as by winding it around a curler before or after it was rendered pliable, and then removing the pliability whereby the hair becomes set in its curled shape. Two general techniques are currently used to impart a change in hair configuration. The first is referred to as permanent (cold) waving. This involves winding the wet hair around so-called waving rods and subjecting the wound up hair to a chemical which cleaves the disulfide bonds. After the desired hair configuration has been achieved, thus completing this process, another chemical is employed to reestablish the disulfide bonds or other cross-links. The resultant wave (or curl) is not removed by shampooing or other cosmetic treatments such as hair coloring or bleaching.
The second technique is referred to as temporary or cohesive setting of hair. In contrast to permanent waving which involves chemical modification of hair, temporary setting manipulates only the moisture content of the hair to attain changes in fiber geometry. This can be readily accomplished either by wetting the hair, winding it on curlers or rollers and allowing it to dry, or alternatively, by employing heated rollers or curling irons as setting implements with dry hair. The set attained in this way, while somewhat stable to ambient humidity is lost completely on wetting or shampooing.
In the case of hair setting employing heat, it has been found that slight amounts of water will improve the pliability of hair and thus improve the quality and lasting of the curl. There is a critical threshold of the quantity of added water and exceeding that threshold produces no additional benefits and indeed, may result in set deterioration. It is also desirable to minimize the amount of water employed because any excess water would have to be driven out of the hair before it can be removed from the curler or the curling iron, since only dry hair will maintain a satisfactory curl. Moist hair, or hair exposed to considerable humidity, will loose a water-wave curl rather rapidly.